1. Field of the Invention
This invention relates to fasteners, and more particularly to threaded fasteners having threads that diminish the likelihood of the fasteners becoming loose after installation.
2. Description of Related Art
With presently available screws that are used to fasten thin metal materials together, the difference between drive torque and strip torque is typically very small. Due to this small differential, it is difficult to install these screws with an effective clamping torque range using a power screwdriver. During installation, tapping torque and clamping torque is generated by friction from the flank portion and root of the thread contacting the thin metal. When the fastener is tightened, the hole in the thin metal begins to enlarge from the root of the thread, and torque beings to drop. This results in many screws being installed with a low clamping force. In applications where the fastener is secured in wood, the wood fibers shrink and lose their ability to hold the thread engagement securely. This results in fastener back-out, which causes failure of the fastened joint. Many different forces affect a screw fastened joint during service, and low clamping torque is the primary reason screws loosen. A threaded fastener that loosens creates a condition that adversely affects the performance of joints and causes fastener and joint failures.
U.S. Pat. No. 2,177,004 to Purtell discloses a screw having locking means formed in the thread, whereby the screw is effectually prevented from loosening when threaded into a companion member.
U.S. Pat. No. 3,661,194 to Macfarlane et al. discloses a prevailing torque fastener of the type that generally includes a threaded section having axially extending projections spaced along a flank of the thread, which projections engage the thread of a mating member with an interference fit.